An Si substrate which is a semiconductor single crystal substrate has excellent crystallinity, and an area of the substrate can be enlarged, so that the substrate is suitable as a substrate to prepare an electronic device, and is used for a general purpose. Examples of the electronic device to be laminated and prepared on the Si substrate include a dielectric device, a piezoelectric device, a ferroelectric device and a pyroelectric device. A buffer layer as an underlayer to be formed on the Si substrate in order to improve a crystal structure of the device has been investigated.
In the buffer layer, as a material excellent in lattice matching with Si crystals of the substrate and lattice matching with crystals of a perovskite oxide constituting the electronic device, ZrO2 formed into an epitaxial film, or stabilized ZrO2 doped with Y is used.
As such a buffer layer, specifically there are reported a laminated thin film (Patent Document 1) using a thin film of zirconium oxide or stabilized zirconium oxide stabilized with Y, and a piezoelectric element (Patent Document 2) using Y-stabilized zirconia (YSZ).
However, in the buffer layer disclosed in Patent Document 1, laminated films having crystal surfaces thereof aligned with the surface of the substrate are formed by an electronic beam evaporation process, and this is not suitable for forming films having a large area. Therefore, establishment of such a film forming technology as to form the buffer layer having the large area is demanded.
Moreover, the buffer layer obtained as a conventional epitaxial film has a large content of rare earth elements including Sc and Y in stabilized ZrO2. In addition, when the film having a large area of 3 inches or more is formed, there is a problem that fluctuations of an in-plane composition increase. For example, a content of Y in a bulk single crystal of Y2O3—ZrO2 is 3.7% at maximum, whereas a content of Y in the epitaxial film is 10% or more. It is difficult to reduce the content of Y in the resultant film. In general, when the film is formed by a sputtering process, a composition of a target is not reflected in that of the epitaxial film as it is, and it is very difficult to find an optimum composition of the target for obtaining the epitaxial film having a targeted composition.
Therefore, the epitaxial film containing excessive yttrium oxide with respect to zirconium oxide has a crystal of tetragonal system attributed to Y2O3, and (100) orientation and (001) orientation are mixedly present in the film. The thin film having single crystallinity is not obtained.
It is to be noted that in Example 1 of Patent Document 3, “an epitaxially grown film of (Y2O3)x(ZrO2)1−x (x=0.04) on the Si substrate” is disclosed.
(Patent Document 1) Japanese Patent Application Laid-Open No. H09-110592
(Patent Document 2) Japanese Patent Application Laid-Open No. 2002-234156
(Patent Document 3) Japanese Patent Application Laid-Open No. H02-082585